hbook: missing h2d binning accessor

Created by: mughetto

Unless I've missed it, it's impossible to loop on H2 bins like we can for H1s:

func tester(sig *hbook.H1D, bkg *hbook.H1D) {
        for i, bin := range sig.Binning().Bins() {
                fmt.Println(i,bin)                              
        }
}

A simple accessor like below seems missing for h2d: https://github.com/go-hep/hep/blob/master/hbook/h1d.go#L89